Much of the following background material is found in REPORT ON 2,4,5 -T, A Report of The Panel Of The President's Science Advisory Committee, Executive Office of the President, Office of Science and Technology, March 1971.
The herbicide 2,4,5-trichlorophenoxyacetic acid, commonly known as 2,4,5-T has the chemical formula: ##STR1##
The usual starting material in the chemical synthesis of 2,4,5-T is 1,2,4,5-tetrachlorobenzene which can be reacted with methanol and sodium hydroxide in an autoclave under high temperature and pressure conditions to give the sodium salt of 2,4,5-trichlorophenol: ##STR2## 2,4,5-trichloroanisole is presumed to be an intermediate in this reaction. The high temperature and high pressure conditions of this step are also favorable for the production of a variety of other compounds from these starting materials. The choice of the proper temperature and pressure, and the control of these conditions throughout the reaction are critical for minimizing side reactions and hence impurities in the final product.
One trace impurity produced in the manufacture of 2,4,5-T has received considerable attention. Tetrachlorodibenzo-p-dioxin, commonly known as TCDD ##STR3## IS PRODUCED AS A SIDE PRODUCT UNDER THE CONDITIONS OF Step (1). This compound is known to be extraordinarily toxic. Therefore, while TCDD generally appears in only minute amounts in 2,4,5-T, its presence even in such small amounts has generated extreme concern.
A clue to the possible origin of the dioxins was suggested in their synthesis by condensation, Tomita, M., Veda, S. and Narisada, M. Yakugaka Zasshi, 79:186, 1959. Two molecules of 2,4,5-trichlorophenol condense directly to give TCDD: ##STR4## Two molecules of 2,4-dichlorophenol condense to give the dichlorodibenzo-p-dioxin, ##STR5## and mixtures of various isomeric chlorinated phenols give mixed chlorinated dioxins. Thus, corresponding dioxins can be formed in the preparation of other herbicides related to 2,4,5-T, for example 2,4-dichlorophenoxyacetic acid, commonly known as 2,4-D. The members of the dioxin family are considered dangerous, in general, and therefore their removal from herbicides or the suppression of their formation in the first place has received considerable attention. Progress has been made particularly in varying conditions in the manufacturing process so that it has been possible to produce 2,4,5-T having levels less than 1ppm. of dioxins. Nevertheless, a need exists for a method of further reducing dioxin levels and for purifying samples containing dioxin in amounts substantially greater than 1 ppm. A particular need exists for a method for reducing dioxin levels in 2,4,5-T, 2,4-D and the like to less than 1 ppm which is suitable for large scale application. The concern over the presence of even small amounts of dioxins on the order of 1 ppm arises both because of the high toxicity of the dioxins and the fact that dioxins tend to be fat soluble and non-biodegradable. Thus, dioxins tend to concentrate in animal fatty tissue.
Incident to our work in removing halogenated aromatic compounds from liquids by chromatographic techniques, it was observed that dioxins, and TCDD in particular, are irreversibly adsorbed on coconut charcoal. Subsequently, it was found that a variety of polynuclear aromatic compounds including dichloro-, trichloro-, tetrachloro- and octachloro-dibenzo-p-dioxins, chlorinated naphthalenes and dibenzofurans are also irreversibly adsorbed on coconut charcoal. The irreversible adsorption on coconut charcoal is not entirely understood but it has been found that the foregoing materials cannot be recovered when adsorbed on coconut charcoal by using extended solvent extraction or displacement.
It is known that some charcoals will preferentially adsorb aromatic organic compounds and this is attributed to the electronic affinity of the aromatic ring for the surface of carbon, in the form of graphite in particular. Synder, R., Principles of Adsorption Chromotography, Marcel Dekka, Inc., New York, 1968, pp. 168-169. However, such adsorption is usually reversible and is typified by our experience with Darco charcoal which reversibly adsorbs the polynuclear aromatic compounds mentioned above which are irreversibly adsorbed on coconut charcoal.